Believe me im level 23 zero champion weapons bought and i still have good skills especially since level 22-26 players actually put up a fight unlike level 17-21 players troll around (not trying to bring people down just you have those hackers that think theyre good)
With a max efficiency of 75 (Adamants max 65), the Champs are technically better, but it's all down to your style of play and how you mix and match your setup...
I'm carrying a Champion Peacemaker and an Adamant Bomber myself. Loving both, even if the ChPM has been replaced by my new Future Police rifle quite a bit lately. Will probably switch back more often now that I am close to maxing out the ChPM (@69).
Both sets are reskins of older/weaker weapons IIRC, and while the Champs can't be bought atm, at least the original Anti Hero rifle, Solar Power cannon and Electric Arc can all be upgraded to 65 efficiency now (although I believe that the Champs have better Mobility and perhaps different Capacity).